1. Field of the Invention
This invention relates to radio communication systems and methods, and more particularly relates to interference cancelling systems and methods for minimizing or eliminating interference in radio receivers due to unwanted signals. Even more specifically, this invention relates to an interference cancelling system and method for minimizing an interfering signal which arrives at the radio receiver through multiple paths due to reflections or when the receiving antenna system is an array.
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional interference cancelling system. The desired signal S.sub.D received by a receiving antenna 2 coupled to a radio receiver 4 is contaminated or interfered with by an interference signal S.sub.I transmitted by transmitter 6 through transmit antenna 8. In accordance with conventional techniques, a sample of the interference signal S.sub.I is taken at Location A by using a directional coupler 10 or power splitter or, alternatively, by using a directive antenna (not shown), especially if the transmitter is remotely located from the receiver. The sample signal S.sub.s is provided by transmission link 12 to subtractor means 14, such as a 180.degree. hybrid or the like, at Location B, where it is subtracted with equal amplitude from the signal S.sub.R received by the receiver 4, thereby nulling the interference signal's effect on the received signal S.sub.R.
In order for the cancellation to take place, the sample signal S.sub.s must arrive at Location B in the same phase as the interference signal S.sub.I. Such can be achieved with a variable phase shifter (not shown) in series with the transmission link 12 which defines the sample signal path between Locations A and B. However, if this phase relationship is to be attained over a defined bandwidth, then the time delay between Locations A and B in the interference path (that is, the path which interference signal S.sub.I follows) must be matched by the time delay in the sample path (i.e. between Locations A and B). A time mismatch of .DELTA.T will cause a phase difference, .DELTA..phi., given by the following equation: EQU .DELTA..phi.=2.times..pi..times.F.sub.o .times..DELTA.T
where F.sub.o is the frequency of the interference signal S.sub.I and .pi. is approximately equal to 3.14. As stated above, the overall phase difference .DELTA..phi. can be made to approach zero by using a phase shifter in the sample signal path.
However, at another frequency, F, there will still be a phase difference, as shown in the following equation: EQU .DELTA..phi.=2.times..pi.(F-F.sub.o).times..DELTA.T
Even though good cancellation may be achieved at frequency F.sub.0, the cancellation may be only partial at frequency F, or there may be no cancellation or even an enhancement of the interference signal S.sub.I received by the radio receiver.
This is illustrated by FIG. 2, which is a graph of the degree of cancellation as a function of frequency for a time mismatch of 1.0 nsec, as well as for 0.1 nsec. In the example shown in FIG. 2, the phase has been adjusted so that at frequency F.sub.0, full cancellation of the unwanted signal, interference signal S.sub.I, occurs. At frequencies away from F.sub.0, the degree of cancellation is reduced. For example, at about 160 MHz or more from frequency F.sub.0, the interference signal S.sub.I is actually enhanced when the time mismatch is 1.0 nsec or greater. Accordingly, it is essential that there be a time match apart from amplitude match in achieving a high degree of cancellation across a defined bandwidth in an interference cancelling system. Such time match is achieved by using an attenuating means 16, such as a pad, and a delay line 18, both being in series with the sample path defined by the transmission link 12 between Locations A and B, as shown in FIG. 1.
A problem occurs when the interference signal S.sub.I follows multiple paths, which can occur due to reflections or when the receiving antenna system is an array. The interference signal S.sub.I is received by the radio receiver after multiple time delays and generally in different strengths due to the multiple paths that it follows. In such a case, the bandwidth of the interference cancelling system can be greatly degraded, since no single delay line and single attenuator can simulate multipath effects. A similar situation occurs when the receiving antenna system is an array whose elements are usually not time-matched with respect to the source of interference.